1. Field of the Invention
The present invention relates to a rotating polygon mirror used in a laser scan type optical system, and more particularly to a rotating polygon mirror which prevents contact of mount portions of the rotating polygon mirrors when a plurality of rotating polygon mirrors are stacked with the mounting portions abutting each other, and prevents damages of the mounting portions due to the contact from occurring.
2. Description of the Prior Art
The rotating polygon mirror is mounted on a laser scan type optical system to change a light path of a laser beam so that the laser beam scans a photosensing surface. Typically, it is formed by polishing or precisely grinding sides of polygon bar made of optical glass or metal such as aluminum to form reflection mirror surfaces.
Such a rotating polygon mirror has continuous upper and lower planes 1 and 2 as shown in FIG. 1A or offset upper and lower surfaces 3 and 4 as shown in FIG. 1B.
However, when a number of such rotating polygon mirrors are stacked with their upper and lower surfaces abutting each other during manufacture, storage or transportation, dust may be held between the surfaces of the stacked mirrors and the stacked surfaces are damaged, or the stacked surfaces are rubbed by each other and the stacked surfaces are damaged. As shown in FIG. 2, when the rotating polygon mirror 10 is mounted to a rotating device shaft 12 of a scan motor 11 of a scan device by a screw 13 with the upper surface 1 or the lower surface 2 of the rotating polygon mirror 10 being set as a reference plane, if mount portions of the rotating polygon mirror 10, that is, the portions to be held by mounting members 14 and 15, particularly a reference surface which should be precisely normal to the reflection mirror plane has the damage, a scan plane angle precision is lowered. Accordingly, it is necessary to prevent such damage.
In the rotating polygon mirror of the above construction, when a coating is applied to the mirror surface by vapor-depositing Al, Cu, Au, Ag, SiO.sub.2, MgF.sub.2, or Al.sub.2 O.sub.3 in order to protect the mirror surface and/or enhance a reflection function of the surface, a plurality of rotating polygon mirrors 18 are set on a vapor deposition jig shaft as shown in FIG. 3. In order to prevent damage of the rotating polygon mirrors and excess deposition of evaporated materials on the upper and lower surfaces of the rotating polygon mirrors, spacers 20 must be arranged between the respective rotating polygon mirrors. As a result, the number of mirrors set in one vapor deposition process is limited because of a limited space in the evaporation bath. That is, the space in the evaporation bath is partially occupied by the spacers.